An inverter’s reactive-power action changes the network voltage and the current passing through relevant equipment. The useful direction depends on the operating point and network. A controller should evaluate the resulting point rather than assuming any voltage-support command also creates transformer thermal margin. [1] [2]
Draw the current constraint at the actual voltage
For balanced sinusoidal three-phase operation, apparent power is S = √(P² + Q²) and I = S/(√3 V). Use P in watts, Q in vars, V as line-to-line RMS volts and I in amperes. At fixed P and V, increasing the magnitude of net transformer Q increases current. Local compensation can instead reduce net Q; the measurement boundary and sign convention determine which case applies.
Evaluate the device’s own apparent-power and current capability as well. A permitted transformer point is not automatically a permitted inverter point. Resource availability, power priority and control permissions can narrow the usable operating region.
| Candidate action | Electrical effect to evaluate | Additional constraint |
|---|---|---|
| Reactive-power adjustment | Voltage response and net Q at the transformer | Device current/apparent-power capability |
| Active-power reduction | Changed real-power flow and network voltage | Energy or export-service requirement |
| Combined P–Q dispatch | Movement within the network’s feasible region | Phase loading and transformer thermal history |
| No further available response | Constraint remains despite a command | Fallback and visible infeasibility |
Use a network calculation for the voltage boundary
The current circle alone cannot establish the voltage response. Use a suitable network representation with the relevant impedances, topology and operating conditions. Confirm the predicted direction and magnitude with available observations. A reactive command that helps one location can affect another location differently.
Intersect the acceptable voltage region with the transformer’s current envelope and the thermal trajectory permitted for the interval. Under material unbalance or harmonic conditions, replace the balanced simplification with the appropriate phase-specific assessment. The retained assumptions should identify where the first-pass plot stops being useful.
Compare alternatives using the same service objective
If reactive support consumes the remaining current margin, compare a feasible active-power adjustment or another available action. Keep the customer or export obligation visible. An option that appears superior after reducing the service request has changed the comparison.
Assume balanced 400 V line-to-line operation, P = 300 kW and transformer current limit 500 A. At Q = 100 kvar, S = 316.23 kVA and I = 456.44 A. Increasing net Q to 200 kvar raises S to 360.56 kVA and I to 520.42 A, exceeding the assumed current limit by 20.42 A. With P unchanged, the ideal current-circle limit is |Q| ≤ √[(√3 × 400 × 500)² − 300,000²] = 173.21 kvar. Limit the voltage-support dispatch accordingly, then apply the actual unbalance, harmonic and thermal constraints.
Compare the requested operating move with achieved local measurements. A voltage-model error, a resource-capability limit and a thermal limit imply different next actions. Identify the boundary that actually stopped the move before changing the optimizer or promising more flexibility.
References
- A. Jain and M. Karimi-Ghartemani, Mitigating Adverse Impacts of Increased Electric Vehicle Charging on Distribution Transformers, Energies 15, 9023, 2022. DOI: 10.3390/en15239023.
- IEC, IEC 60076-7:2018, Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.